(1) Field of the Invention
This invention relates to cold rolling oils (hereinafter referred to simply as "rolling oil") for steel sheets which are applied for cold rolling of steel sheets and have excellent lubricity, lubrication stability, and fresh oil replenishing property.
(2) Description of the Prior Art
Rolling oils are prepared by adding various emulsifying and dispersing agents to mixtures obtained by adding oiliness improvers, extreme-pressure additives, antioxidants and the like to animal or vegetable oils such as tallow, palm oil and the like, various synthetic esters, mineral oils, or the mixed oils thereof. In rolling, a liquid obtained by emulsifying and dispersing a rolling oil in a suitable concentration (hereinafter referred to "coolant liquid") by means of mechanical agitation in a tank (hereinafter referred to "coolant tank") is sprayed on work rolls for cooling and the surface of steel sheets for lubricating oil, and is circulated.
For the sake of elevating productivity, it is recently intended to perform high-speed rolling and continuous manufacturing of steel sheets. In this respect, it is required that rolling oil has excellent lubricity, and particularly stability of lubrication.
Lubricity and stability in lubrication are affected by composition of a rolling oil, and they are also influenced significantly by extent of and changes in amount of adhesion to the steel sheet (plate-out). More specifically, a less amount of plate-out brings about insufficient lubrication, whilst it causes variation of lubrication even though there is much more amount of plate-out, if there is a variation in such amount so that it results in a lack in uniformity. Accordingly it is preferred for favorable lubricity and stability in lubrication that such amount of plate-out is remarkable and in addition, uniform. Furthermore the amount of plate-out is significantly related to particle size of rolling oil in a coolant liquid to be sprayed (the amount of plate-out becomes small in case of small particle size), so that lubricity is dependent upon particle size of the rolling oil. Such particle size is easily influenced by stirring conditions. In this connection, since a coolant liquid passes through pumps, nozzles and return lines by means of circulation in addition to agitation of the coolant liquid in coolant tanks in case of rolling, the stirring conditions vary. Even under such conditions as mentioned above, it is desired that particle size of the rolling oil is uniform and stable.
Nonionic or anionic emulsifying and dispersing agents have heretofore been employed for rolling oils. On one hand, rolling oil particles exhibit a particle size distribution of wide range extending from 2 to 40 microns, because of formation of finer particles due to agitation and formation of larger particles due to coagulation. Owing to such non-uniformity, the plate-out amount becomes also non-uniform so that lubricity varies easily.
As a result of various studies, such problem could be solved by using a cationic high-molecular compound as emulsifying and dispersing agent. Cationic high molecular compounds have heretofore been utilized for organic substances as coagulant and dispersion stabilizers. It is known that a slight amount of cationic high-molecular compound exhibits a coagulating effect, whilst strong dispersion stabilizing effect is observed in the case when a comparatively large amount of cationic high-molecular compound is employed. This is because an organic substance is negatively charged by means of agitation so that the organic substance charged is electrically adsorbed on cationic high-molecular compound strongly. Further, in the case where a slight amount of cationic high-molecular compound is used, the surface potential of particle is neutralized so that such cationic high-molecular compound exhibits coagulating effect. On the other hand, when a large amount of such cationic high-molecular compound is utilized, the high-molecular compound covers the particles to give positive surface potential thereto so that coagulation is prevented by the resulting electric repulsion effect as well as the steric hindrance effect of the macromolecule, and it exhibits dispersion stability.
When a cationic high-molecular compound is used for rolling oil as the emulsifying and dispersing agent, since such high-molecular compound has excellent coagulation resistance, particles formed in case of vigorous agitation are not coagulated and exit stably, even if agitation force becomes weaker. Furthermore, since the emulsifying and dispersing agent is a high-molecular compound, such compound includes a plurality of fine particles so that comparatively large particles exist. As a result, particle size distribution is narrow and sharp. In this case, particle size can be controlled by structure and molecular weight of a cationic high-molecular compound to be used.
However, cationic high-molecular compound scarcely reduces interfacial tension, although such compound is excellent in emulsion and dispersion stability. For this reason, a cationic high-molecular compound is unfavorable in initial emulsifying and dispersing property so that higher energy than that in conventional cases is required for emulsification and dispersion. Thus, such cationic high-molecular compound does not emulsify and disperse rolling oil in the coolant liquid easily at the time of replenishment thereof so that target concentration is not obtained. As a result, more rolling oil than it requires is replenished, and then such problem that cost of rolling oil becomes high arises. In addition, such trouble that lubricity varies arises, because such oil which has not been emulsified and dispersed, but has floated is involved non-uniformly in the circulating system.